An ethylene.α-olefin random copolymer, so-called linear low-density polyethylene (LLDPE), as compared with high-pressure low-density polyethylene, is excellent in properties such as transparency, stress-cracking resistance, low-temperature heat-sealability, heat-seal strength and impact resistance. With these features taken advantage of, it is widely used as a sealant for food packaging. In particular, LLDPE produced by polymerization by using a single-site catalyst is excellent also in transparency, low-temperature heat-sealability, impurity sealability and hot tack property.
Methods proposed to improve the properties of LLDPE films such as transparency and mechanical strength include a method in which films are biaxially-stretched under specific conditions (Patent Document 1); and a method in which a composition given by adding to LLDPE a high-density polyethylene or a high-pressure low-density polyethylene is biaxially-stretched to provide a shrink film (Patent Document 2).
Patent Document 3 discloses a biaxially-stretched film with excellent tear property composed of a composition given by adding, to LLDPE, a high-pressure low-density polyethylene or a high-density polyethylene.
On the other hand, regarding using for films a long-chain branching polyethylene produced by using a transition metal complex catalyst, Patent Document 4 discloses a film composed of a long-chain branching polyethylene; and Patent Document 5 discloses a film composed of a blend composition containing a long-chain branching polyethylene.
Patent Documents 6 and 7 disclose long-chain branching polyethylenes, which can be contained in the polyethylene composition of the present invention, as well as disclose polyethylene compositions containing the same and films formed thereof. However, these Documents of conventional techniques fail to mention that they can be used for biaxially-stretched films, and have no mention of specific compositions suitable for biaxially-stretched films.